The biggest users of water in the US are power plants, particularly the fossil fuel and nuclear plants that together generate almost 90 per cent of the nation's energy. Nearly half the water the US consumes is used to cool these plants and drive their turbines.

Thirsty power

But some renewable and clean energy sources also need a lot of water, says the DoE report. The main consumer is hydropower, which requires large reservoirs of water to generate electricity. Other technologies use it indirectly, to irrigate biofuel crops, for example, or cool and wash solar panels.

According to the DoE's National Renewable Energies Laboratory, whose figures are cited in the report, a typical hydroelectric power plant uses between 15,000 and 68,000 litres of water per megawatt hour generated, while a typical concentrating solar plant – which uses mirrors to focus sunlight onto a small area – uses about 3000 litres of water per megawatt hour of electricity generated.

The figures also show that a nuclear power plant uses 2650 litres per megawatt hour, whereas a typical coal fired power plant uses 1900 and a natural gas plant 750. More widely used renewable technologies, such as wind farms and photovoltaic solar plants, use virtually no water.

Currently renewable and clean energy technologies only make up a small proportion of the US's energy mix – for example, hydroelectric provides 7 per cent of the country's power, and solar less than 1 per cent – but if the US is going to meet its target of reducing carbon emissions by 80 per cent by 2050, renewables and clean energy will need to feature more heavily in future.

The US got a taste of things to come last August when a period of sustained drought dried up rivers that provide water to the plants, forcing several to shut down or reduce their output. Crops grown for biofuel also suffered.

The UCS's report shows just how tricky the balancing act between reducing emissions and saving water could become. They calculate that if things continue on their current path, with the US relying on natural gas and to a lesser extent on renewable energy from wind and solar, water usage will drop as old coal and nuclear plants are retired. But this scenario won't meet the US's carbon emissions target.

Other energy scenarios could reach that goal, but not all are water-friendly. A combination of increasing nuclear energy and deploying more carbon capture and storage technology, for instance, would require 42 per cent more water by 2050 than the current strategy. Alternatively, if the US uses mostly wind and photovoltaic solar power by 2050 it would use 80 per cent less water, but electricity demand would need to drop for these technologies by themselves to be sufficient.

Ultimately, Rogers says, water scarcity should be taken into account along with emissions when planning new power sources. "As we build the power sector of the future, we're hardwiring risk or resilience into it," he says.

Correction: When this article was first published on 19 July, it cited figures from the DoE's National Energy Technology Laboratory. That has since been corrected to the DoE's National Renewable Energies Laboratory.

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